Lebanon's president Michel Aoun has called on Saudi Arabia to clarify why Lebanese Prime Minister Saad al-Hariri could not return home, a week after he stunned his home country by announcing his resignation while in the kingdom.

A senior Lebanese official said President Michel Aoun had told foreign ambassadors Hariri had been "kidnapped" and should have immunity.

Hariri's resignation, which surprised even his close aides, has plunged Lebanon into crisis. It has thrust the country back into the frontline of a power struggle between Sunni Saudi Arabia and Shi'ite Iran - a rivalry that has wrought upheaval in Syria, Iraq, Yemen, and Bahrain.

"Lebanon does not accept its prime minister being in a situation at odds with international treaties," Aoun said in a statement on Saturday.

He said any comment or move by Hariri "does not reflect reality" due to the questions over his status following his shock resignation in a broadcast from Saudi Arabia.

Lebanese authorities believe Riyadh is detaining Hariri, two top Lebanese government officials, a senior politician close to Hariri and a fourth source have said.

French President Emmanuel Macron, who made an unscheduled visit to Riyadh this week, phoned Aoun on Saturday to discuss the crisis. A French official had made comments suggesting Paris believed Hariri may not be a free man.

Riyadh says Hariri is free and decided to resign because Iran's Lebanese ally, Hezbollah, was calling the shots in his coalition government.

Hariri has made no public remarks since quitting last week, when he said he feared assassination and accused Iran along with Hezbollah of sowing strife in the Arab world.

Hariri, whose family made its fortune in the Saudi construction industry, has also given no sign of when he might return to Beirut.

The Lebanese premier took part in a ceremony in Riyadh on Saturday welcoming Saudi King Salman from Medina, his media office said. Hariri met with the Turkish and British ambassadors at his Riyadh home in the afternoon, it said.

Sources close to Hariri say Saudi Arabia has concluded that the prime minister - a long-time Saudi ally - had to go because he was unwilling to confront Hezbollah.

President Michel Aoun could file a complaint to the UN Security Council if Prime Minister Saad Hariri, who resigned last week from Riyadh, did not return to Beirut and the circumstances surrounding his stay in Saudi Arabia are not clarified, al-Joumhouria daily reported on Saturday.

Aoun has told the diplomatic delegations he met, that he “would not wait more than a week to clarify the fate of Hariri before he refers the file to the international community,” informed sources told the daily.

They added “there are international preparations to assist Lebanon in this endeavor, in an implicit reference to Russian and other Western preparations.”

Baabda circles said that “Aoun was clear and frank in talks with the International Support Group for Lebanon (on Friday) and explained the circumstances that accompanied the resignation.

“He was surprised to hear from the prime minister last Saturday that he can no longer bear the situation and that he will be in Beirut within two or three days. But since then no contact between the two men was made whatsoever.”

Baabda circles elaborated to say that the President has “expressed extreme concern over the circumstances of Hariri's stay in Saudi Arabia, he said “Lebanon awaits for the ISG assistance to secure Hariri's return because what is happening harms the the dignity of Lebanon and the Lebanese.””

Hariri resigned in a shock announcement broadcast from Riyadh on Saturday as a power struggle intensified between Saudi Arabia and Iran, which backs Lebanon's Hizbullah.